1. Field of the Invention
The present invention relates to an exhaust gas purification device for an engine. More specifically, the present invention relates to a device that is capable of effectively removing an NOx component from exhaust gas.
2. Description of the Related Art
An exhaust gas purification device capable of removing an NOx component from engine exhaust gas is, for example, disclosed in the Japanese Unexamined Patent Publication (Kokai) No. 62-117620. The device disclosed in JPP ""620 is used for removing NOx components from the exhaust gas of a lean burn engine, i.e., an engine operated at an air-fuel ratio higher than a stoichiometric air-fuel ratio. In the device disclosed in JPP ""620, an exhaust passage of the engine is diverged into two branch exhaust passages, and a NOx absorbent is disposed in each of the branch exhaust passages, namely, two NOx absorbents are disposed in the exhaust gas passage in parallel. The NOx absorbent is capable of absorbing an NOx component in the exhaust gas in the presence of oxygen. Since the engine is operated on a lean air-fuel ratio, the exhaust gas from the engine contains relatively high concentration of oxygen, therefore, NOx components contained in the exhaust gas are absorbed by the NOx absorbents.
When the NOx absorbent absorbs a certain amount of NOx, the ability of the NOx absorbent to absorb NOx in the exhaust gas is reduced. To regain the absorption ability, the absorbed NOx in the absorbent must be discharged and reduced periodically. For example, by supplying a reducing agent to the NOx absorbent, the NOx absorbents are regenerated to regain the initial NOx absorption ability. In the device in JPP ""620, two NOx absorbents are disposed in parallel in the exhaust passage in order to enable regeneration of the NOx absorbent without stopping the purification of the exhaust gas. In JPP ""620, two NOx absorbents are used alternately during normal engine operation, i.e., only one of the NOx absorbents is used for absorbing the NOx in the exhaust gas. Namely, in the normal operation, one of the branch exhaust passage is closed, and the exhaust gas from the engine is directed to the other branch exhaust passage where the NOx in the exhaust gas is absorbed by the NOx absorbent disposed in that exhaust branch passage. While absorbing NOx in the exhaust gas by one NOx absorbent, the other NOx absorbent disposed on the closed exhaust branch passage is regenerated by supplying a reducing agent, such as hydrogen gas.
According to the device disclosed in JPP ""620, by using two NOx absorbents disposed in the exhaust passage in parallel, the NOx absorbents can be regenerated alternately while continuing to remove the NOx component from the exhaust gas.
However, a disadvantage of the device disclosed in JPP ""620 has is that the reducing agent must be supplied to both exhaust branch passages. This requires two injection nozzles, one in each of the exhaust branch passages, and associated piping connecting the nozzles to the supply source of the reducing agent, and a three-way flow control valve for switching the supply of the reducing agent between the two nozzles. Further, elaborate control of the three-way flow control valve is required to enable the appropriate regeneration of the NOx absorbents.
In view of the problems in the prior art set forth above, the object of the present invention is to provide an exhaust gas purification device for an internal combustion engine which is capable of removing NOx components from the exhaust gas without requiring complicated devices and controls.
The above object is achieved by the present invention, an exhaust gas purification device, for an internal combustion engine. The engine is operated with an air-fuel ratio higher than a stoichiometric air-fuel ratio. The device comprises (a) first and second NOx absorbents disposed in the exhaust passage of the engine and connected in series by an exhaust connecting passage, each of the NOx absorbents absorbs NOx components in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the NOx absorbents is lean, and releases the absorbed NOx when the oxygen concentration in the exhaust gas becomes lower; (b) a reducing agent supply means for supplying a reducing agent to the exhaust gas in the exhaust connecting passage between the first and the second NOx absorbents; (c) a switching means for switching the direction of the exhaust gas flow between a first direction in which the exhaust gas flows through the first NOx absorbent, the exhaust connecting passage and the second NOx absorbent in this order, and a second direction in which the exhaust gas flows through the second NOx absorbent, the exhaust connecting passage and the first NOx absorbent in this order; (d) and a control means for controlling the switching means so that the direction of the exhaust gas flow is switched between the first and the second direction each time when a predetermined condition is satisfied.
When the flow direction of exhaust gas is switched to the first direction, the lean air-fuel ratio exhaust gas from the engine first flows into the first NOx absorbent and NOx components in the exhaust gas are absorbed by the first NOx absorbent. After passing through the first NOx absorbent, a reducing agent is supplied to the exhaust gas in the exhaust connecting passage before flowing into the second NOx absorbent. Therefore, the air-fuel ratio of the exhaust gas flowing into the second NOx absorbent becomes rich due to the reducing agent. Thus, in the second NOx absorbent, the absorbed NOx is released and reduced to nitrogen by the reducing agent. Namely, when the exhaust gas flow is switched to the first direction, NOx components in the exhaust gas are absorbed by the first NOx absorbent and removed from the exhaust gas, and the NOx in the second NOx absorbent is released and reduced to nitrogen.
On the other hand, when the exhaust gas flow is switched to the second direction, the lean exhaust gas from the engine first flows into the second NOx absorbent and NOx components in the exhaust gas are absorbed by the second NOx absorbent. Then, the reducing agent is supplied to the exhaust gas in the exhaust connecting passage before flowing into the first NOx absorbent. Therefore, the rich exhaust gas flows into the first NOx absorbent, and absorbed NOx is released from the first NOx absorbent and reduced to nitrogen by the reducing agent. Therefore, by switching the direction of the exhaust gas flow each time when the predetermined condition is satisfied, for example, each time an amount of NOx absorbed in the NOx absorbent reaches a predetermined value, absorption of NOx in the exhaust gas and releasing and reducing of the absorbed NOx is performed in each NOx absorbent alternatively without stopping the absorption of NOx in the exhaust gas as a whole.
According to another aspect of the present invention, there is provided an exhaust gas purification device for an internal combustion engine having an exhaust passage. The engine is operated at an air-fuel ratio equal to, or lower than, a stoichiometric air-fuel ratio. The device comprises (a), first and second NOx absorbents disposed in the exhaust passage of the engine and connected each other in series by an exhaust connecting passage, each of the NOx absorbents absorbs NOx components in the exhaust gas when the air-fuel ratio of the exhaust gas flowing into the NOx absorbents is lean, and releases the absorbed NOx when the oxygen concentration in the exhaust gas becomes; (b) a secondary air supply means for supplying secondary air to the exhaust gas in the exhaust connecting passage between the first and the second NOx absorbents; (c) a switching means for switching the direction of the exhaust gas flow between a first direction in which the exhaust gas flows through the first NOx absorbents, the exhaust connecting passage and the second NOx absorbent in this order, and a second direction in which the exhaust gas flows through the second NOx absorbent, the exhaust connecting passage and the first NOx absorbent in this order; and (d) a control means for controlling the switching means so that the direction of the exhaust gas flow is switched between the first and the second direction each time when a predetermined condition is satisfied.
In this aspect of the invention, the air-fuel ratio of the exhaust gas from the engine is rich, i.e., lower than, or equal to, the stoichiometric air-fuel ratio, and relatively low concentrations of oxygen and a relatively high concentrations of HC and CO components are contained in the exhaust gas. Therefore, when the direction of exhaust gas flow is switched to the first direction, the exhaust gas with a rich air-fuel ratio flows into the first NOx absorbent, and the absorbed NOx is released from the first NOx absorbent and reduced to nitrogen by HC and CO components in the exhaust gas. After passing through the first NOx absorbent, the secondary air is supplied to the exhaust gas in the exhaust connecting passage before it flows into the second NOx absorbent, and the air-fuel ratio of the exhaust gas flowing into the second NOx absorbent becomes lean. Therefore, a small amount of NOx which passes through the first NOx absorbent is absorbed by the second NOx absorbent and removed completely from the exhaust gas.
Namely, when the direction of the exhaust gas flow is switched to the first direction, the absorbed NOx is released from the first NOx absorbent and reduced to nitrogen, and at the same time, NOx in the exhaust gas is absorbed by the second NOx absorbent.
Similarly, when the direction of the exhaust gas flow is switched to the second direction, the absorbed NOx is released from the second NOx absorbent and reduced to nitrogen, and NOx in the exhaust gas is absorbed by the first NOx absorbent at the same time.
Therefore, in this aspect of the present invention, by switching the direction of the exhaust gas flow each time when the predetermined condition is satisfied, absorption of NOx in the exhaust gas and releasing and reducing of the absorbed NOx is performed in each NOx absorbent alternatively, without stopping absorption of NOx in the exhaust gas as a whole.